One of the preferred methods for making electronic microcircuits is with thick film technology. Thick film circuits are made by alternatively applying layers of conductor and dielectric to a substrate which is usually alumina. The layers of conductor are connected to each other through small holes in the dielectric filled with conductor. These holes are called vias. The layers are made by printing a thick film paste on a screen printer, drying the layer at low temperature (&lt;200.degree. C.) to remove volatile solvents and then firing the layer at high temperature (&gt;600.degree. C.) to remove all other noninorganic components and densify the inorganic components. Frequently several layers are printed and dried, then fired together. This is called cofiring.
Thick film pastes are dispersions of inorganic solids in a liquid vehicle phase. This vehicle phase contains predominantly polymers, solvents and other miscellaneous additives. The inorganic solids can be conductive (e.g., gold, silver, copper) or dielectric (e.g., glass, refractory oxides). The polymer in a thick film paste serves at least the following functions: imparts proper printing rheology; provides good adhesion of the dried print to the substrate (alumina or thick film dielectric) before firing and gives enough strength to the unfired composite so it will not crack or chip during handling prior to firing.
New electronic circuits are being designed with finer features so more functionality can be placed in smaller spaces and/or higher circuit transmission speeds can be attained. Circuit makers have also required faster printing speeds to produce circuits at lower cost and in higher volumes. To manufacture these new circuits using thick film technology, dielectric pastes have to print smaller vias (openings between conductor layers) and conductor pastes will have to print finer lines. New generations of thick film paste products need to print with this higher resolution, but also at faster print speeds to meet these market needs. The vehicle (polymer, solvents, other organic additives) controls these printing parameters to a very large extent.
The polymer most commonly used in thick film pastes is ethyl cellulose. The ability to modify printing is restricted if ethyl cellulose is used because the printing is controlled to a great extent by the rheological properties imparted by the polymer to the paste. Most pastes are shear thinning (decreasing viscosity with increasing shear). The rate of recovery of the paste viscosity after the shear induced by the printing process is limited to a narrow range for pastes containing ethyl cellulose. To get resolution of fine features with these pastes, the viscosity of the paste has to be maintained at a high level. Pastes with viscosity in this range can not be printed at high speeds because the viscosity does not get low enough fast enough for efficient transfer of paste through the screen. The paste also tends to be tacky, causing the screen to stick to the printed part, especially at high print speeds. If the viscosity recovery rate of the paste could be controlled independently of the equilibrium viscosity, then much greater latitude would be possible in formulating high resolution, fast printing pastes.